Analysis of Residual Stresses in Laser-Shock-Peened and Shot-Peened Marine Steel Welds
- PDF / 4,469,395 Bytes
- 12 Pages / 593.972 x 792 pts Page_size
- 16 Downloads / 173 Views
RODUCTION
COMPRESSIVE residual stress has a beneficial effect on fatigue life. For surface treatments aimed at inducing a compressive residual stress, key parameters include the magnitude and the depth of the compressive stress. Conventionally shot peening has been used to improve the fatigue life of structural members. Laser shock peening (LSP) is a relatively new technique that is already being deployed widely for aeroengine components, and that is being optimized with regard to process parameters for its application to different materials. Laser peening uses a high power density laser beam that is pulsed on to a metal surface that is covered by a water layer and which may also be protected by paint or tape with thickness around 100 µm[1] which then acts as an ablative layer, to protect the metal surface from thermal effects.[2] The laser energy vaporizes the surface layer to form a plasma. The pressure of the plasma rises as the laser pulse continues, and it is confined by the water layer to create a shock wave that plastically strains the near-surface material.[3] The elastic relaxation of the surrounding material then forces the surface material into compression. The depth of plastic deformation and the resulting compressive residual stress is significantly greater than most other surface treatment techniques. Laser peening imparts compressive residual stress to a BILAL AHMAD, Research Associate, and MICHAEL E. FITZPATRICK, Professor, Lloyd’s Register Foundation Chair in Structural Integrity and Systems Performance, formerly with the Department of Engineering and Innovation, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK, are now with the Centre for Manufacturing and Materials Engineering, Coventry University, Priory Street, Coventry CV1 5FB, UK. Contact e-mail: michael. fi[email protected] Manuscript submitted May 26 2016. Article published online November 21, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
depth of 1 to 4 mm and the near-surface magnitude of the residual stress can approach the material’s yield strength. Multiple layers of peening are commonly used to ensure a uniform stress distribution, with subsequent layers offset to the first layer.[2,4] While some early studies on laser peening implied that the absence of an ablative layer would always lead to tensile residual stress at the surface of a sample, more recent work has shown that this is not necessarily the case, and surface compression can be obtained even in the absence of an ablative layer.[5] Shot peening is the process of bombardment of a surface with small spherical media called shot. The shots are usually made of steel, glass etc., and the diameter of shot is typically 0.5 to 1.5 mm. Shot peening involves multiple and repeated impacts. Each shot striking the metal yields the material in tension, and when the elastically strained material below the surface relaxes it pushes the surface material into compression. The magnitude of compressive stress is directly related to the yield strength of the base material, and typical
Data Loading...
